I get hideously sick just going backwards on a train so wouldn't inflict that on my child.
I do as well, but my children don't so I don't think it's cruel to try it out. Many seats these days go easily from rear facing to forward facing so you can always change if it causes a problem.
Also... how do families afford all these seats?
The most expensive seats are the ones for children under 4 which have been required for over a generation now (well it used to be up to age 3). In the 90s looking back at Argos catalogues etc these were priced around £60, which is about £135 today - I would say they have got more expensive, as most people spend around £200-300 on a car seat for this stage, but on the other hand, you can certainly get car seats around the £100-150 mark, some basic ones even cheaper, and the mark up is likely due to increased marketing, influencers, newer features such as isofix and spinning, which started out as luxury features but are now considered "must-haves". (Consider the fact that a high end but still mainstream pushchair now costs over £1000 - new parents are extremely big money and those who have it are willing to spend it).
What also happened when that rule change came in requiring them to be used up to 12 years (which was 2006, so not that recent) was that a lot of companies started bringing in seats which do the toddler stage with the 5 point harness and then at about 3-4yo you take the harness out and it becomes a high back booster seat, rather than needing a separate one. In practice as well it's only one extra seat that you need and it replaces the backless booster seat, which some people did use back before they were required by law. You can even still legally use a backless booster, if you really want to - it's just not recommended because a high back one offers better protection. In future this won't be legal any more because new backless boosters have a minimum weight and height consistent with a 5-6 year old, but for now there are plenty of backless boosters on the market with a 15kg (3yo) minimum and probably will be for quite a long time.
Then when the rear facing seats started to become more popular due to the internet and being sold in normal shops rather than imported as a fringe thing you started to see seats which claim to do everything - rear facing up to about 2 or 4, forward facing up to 4, high backed booster. In reality they tend to be overpriced and not great at any of the stages but in theory, you could do it all birth-12 years with one single £200-300 outlay. (I think this is a false economy personally, since you can get the cheapest possible seats birth-4 for about £50, backless booster £10 or high back £25. Or decent seats birth-4 about £100, high back about £50, add £40 for an infant carrier if desired.)